It has previously been proposed to determine the mass flow rate of a fluid medium, typically air, by placing an impediment in the stream of the fluid, and vibrating the impediment. The impediment may, for example, be a plate or the like located transversely with respect to the direction of flow of the fluid. This plate is vibrated by a vibratory element, which may be a piezoelectric transducer, an induction coil, or the like. The vibrations of the plate are damped due to the pressure of the fluid medium thereagainst. Damping of the vibration can be sensed and will be a measure of the mass flow rate of the fluid.
Mass flow measuring elements based on this construction, as well as evaluation circuits for use therewith, are known--see, for example, German Patent Disclosure Document DE-OS No. 29 28 568. As described therein, the vibratory element is controlled by impulse width modulation, which requires substantial circuitry. This increases the cost of the mass flow meter, and permits its use only in very restricted and special situation, where the advantages of the sensitivity thereof outweight the costs. In accordance with this publication, an impediment is located within a tubular duct in which the medium flows, the impediment causing backing-up or damming of the fluid. The impediment, typically a plate, is vibrated by a piezoelectric drive. The interaction of the vibration of the plate with the fluid flowing in the duct causes damping of the body which is proportional to the mass-flow rate, that is, to the mass flow per unit time. Digital or analog output of the mass flow can be obtained.